1. Field of the Invention
The present invention relates to a process for electrochemical separation of oxygen from oxygen containing gaseous mixtures, such as air, utilizing a molten nitrate salt electrolyte retained in a matrix between two electrodes, wherein oxygen is separated from the gaseous mixture when electrical potential is applied across the electrodes.
2. Description of the Prior Art
Relatively pure oxygen gas has many industrial and medical uses. One process to produce oxygen is electrolysis of water. Electrolysis consumes large amounts of electrical energy and has the further disadvantage of the co-production of hydrogen which presents safety and purity problems.
One widely used oxygen separation process involves cryogenic liquefaction and distillation of air. Cryogenic distillation processes are generally energy intensive and operate at overall efficiencies of less than about 35-40 percent. Cryogenic distillation is generally not economically feasible unless it is operated in very large scale plants, and large scale production results in additional freight costs from a centralized production facility to the end user.
Chemical oxygen separation processes have been developed, such as the Moltox chemical air separation process marketed by Air Products and Chemicals, Inc. This chemical air separation technology claims to achieve reduced energy consumption and therefore increased efficiency, as compared to cyrogenic processes. The basic Moltox chemical air separation process and improvements thereto are described in the following U.S. patents: U.S. Pat. No. 4,132,766 teaches separation of oxygen from air by a regenerative chemical process wherein air is contacted with a molten alkali nitrite and nitrate salt solution oxygen acceptor at elevated temperatures and pressures, causing oxygen to react with nitrites, thereby forming additional nitrates in the molten salt solution. The oxidized molten salt is separated from the oxygen depleted air, and its pressure is reduced while its temperature is increased, causing the release of oxygen. The regenerated oxygen acceptor may then be recycled and the air separation process may be operated in a continuous mode. Separate reactors are required for the absorption and desorption stages, since they are carried out at different temperatures and pressures, requiring pumping of the molten salt oxygen acceptor between the reactors. Corrosion is a serious problem, particularly at the required process temperatures of about 530.degree. to 930.degree.. U.S. Pat. No. 4,340,578 teaches an improvement of the chemical air separation process of the '766 patent, wherein oxygen absorption is conducted in multiple countercurrent stages. Isothermal and adiabatic compression is combined to reduce the compression energy requirement, and the exhaust is processed in a combustion, partial expansion, heat exchange, and completion of expansion sequence to increase the recover of compression energy. U.S. Pat. No. 4,287,170 teaches another improvement of the chemical air separation process involving production of oxygen and nitrogen by air separation using an oxygen acceptor such as molten alakali nitrite solution, SrO, or Pr-Ce oxides, with the remaining oxygen being removed by reaction with a scavenger such as MnO to produce an oxygen-free nitrogen-argon mixture. The oxygen acceptor and oxygen scavenger are regenerated and recycled. U.S Pat. No. 4,526,775 teaches another improvement of the chemical air separation process wherein multiple absorption-desorption cycles are utilized to reduce power requirements and capital costs and increase high pressure oxygen recovery. U.S. Pat. No. 4,529,577 teaches a further improvement to the chemical air separation process wherein a molten salt anion composition includes combined peroxides, oxides and superoxides present in less than about 1 mole percent based upon sodium peroxide, to reduce the corrosiveness of the molten salt solution. U.S. Pat. No. 4,565,685 teaches a further improvement of the chemical air separation process wherein a temperature swing absorption-desorption cycle is used in combination with a pressure swing wherein the pressure is eleveted in the desorption stage to provide more efficient generation of high pressure oxygen.
Other chemical processes for separating oxygen from air include those taught by U.S. Pat. No. 1,120,436 which teaches a chemical separation process wherein air reacts with a lower oxide of nitrogen, such as nitrous anhydride (N.sub.2 O.sub.3) to form a higher oxide of nitrogen, such as nitric acid which, upon heating, decomposes to release oxygen and a lower oxide. Sulfuric acid is used as an intermediary to aid in the oxygen separation; U.S. Pat. No. 4,089,938 teaches an oxygen separation process wherein air is contacted with a suspension of manganese dioxide in an aqueous solution of sodium or potassium hydroxide in a lower pressure absorbing zone, and the resulting liquid oxygen enriched stream is then pumped to a high pressure generating zone and contacted with steam to release the absorbed oxygen; and European patent No. 98,157 teaches a solvent absorption system for separation of oxygen utilizing temperature and/or pressure swings to maintain the necessary oxygen pressures during absorption and desorption.
Separation of oxygen from a mixture of gases such as air by electrochemical means has also been proposed. East German Pat. No. 119,772 teaches recovery of oxygen enriched air using high temperature electrolytic cells having solid zirconium oxide electrolyte operated at 1200.degree.. The solid electrolyte is provided with porous layers of LnCoO.sub.3 (Ln=rare earth) on both the anode and cathode sides. U.S. Pat. No. 4,061,554 discloses chemical oxidation of air to form a peroxide which is electrochemically oxidized to evolve oxygen and regenerate a reduced form which is recycled to the chemical oxidation reactor. U.S. Pat. No. 4,300,987 teaches production of oxygen from air in an aqueous alkaline electrolyte wherein formed peroxide is catalytically decomposed. U.S. Pat. No. 3,410,783 teaches separation of oxygen from air using an electrochemical cell with an aqueous electrolyte which is transported to a separator maintained under a pressure differential relative to the gaseous cell input for oxygen separation. U.S. Pat. No. 3,888,749 teaches electrolytic separation of oxygen from air without application of an external current by having two cells with an aqueous electrolyte circulated between them, the first cell having a high oxygen partial pressure and the second cell having a low oxygen partial pressure producing an emf between the cells and liberating oxygen from the electrolyte in the low oxygen pressure cell. U.S. Pat. No. 4,475,994 teaches an electrochemical process for separating oxygen from a mixture of gases wherein oxygen is reduced to the superoxide ion O.sub.2.sup.- at the cathode, transported by the electrolyte to the anode, and is there reoxidized to oxygen and collected. Aqueous electrolytes at high pH, non-aqueous electrolytes, and solid polymer electrolytes may be used in the practice of the '994 invention. Nitriles, Lewis acids, organic cations, macromolecules such as crowns and cryptands and/or ligands may be added to stabilize the superoxide ion in an aqueous electrolyte.